Polycarbonate films prepared by coating methods

ABSTRACT

A method of film fabrication is taught that uses a coating and drying apparatus to fabricate resin films suitable for optical applications. In particular, polycarbonate films are prepared by simultaneous application of multiple liquid layers to a moving carrier substrate. After solvent removal, the polycarbonate films are peeled from the sacrificial carrier substrate. Polycarbonate films prepared by the current invention exhibit good dimensional stability and low birefringence.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This is a 111A Application of Provisional Application, Serial No.60/381,931, filed on May 20, 2002.

FIELD OF THE INVENTION

[0002] This invention relates generally to methods for manufacturingresin films and, more particularly, to an improved method for themanufacture of optical films, and most particularly, to the manufactureof polycarbonate films used as substrates, polarizer plates,compensation plates, and protective covers in optical devices such aslight filters, liquid crystal displays and other electronic displays.

BACKGROUND OF THE INVENTION

[0003] Polycarbonates (PC) are used to produce films that are noted fortheir transparency, mechanical strength, and thermal stability. As aresult, polycarbonate films have a number of optical applications. Inparticular, transparent polycarbonate films have recently been suggestedfor use as protective covers for light polarizers, as polarizer sheets,as compensation plates, and as electrode substrates in optical displays.In this regard, polycarbonate films are intended to replace glass andless stable polymeric films to produce lightweight, flexible opticaldisplay screens. These display screens may be utilized in liquid crystaldisplays, OLED (organic light emitting diode displays, and in otherelectronic displays found in, for example, personal computers,televisions, cell phones, and instrument panels.

[0004] Polymers of the polycarbonate type are available in a variety ofmolecular weights as well as in numerous permutations around the basicmolecular structure. Common to all polycarbonates are the carbonatelinkages and usually the presence of stabilizing phenyl groups (Ph) inthe polymer backbone. In terms of commercially significantpolycarbonates, the condensation product of the dihydridic phenol,2,2-bis-(4-hydroxyphenyl)-propane (Bisphenol-A), with a carobonateprecursor such as phosogene forms a polymer having recurring units of-O-Ph-C(CH₃)₂-Ph-O-CO-. Polycarbonates of the Bisphenol-A type are bothreadily available and relatively inexpensive.

[0005] In general, resin films are prepared either by melt extrusionmethods or by casting methods. Melt extrusion methods involve heatingthe resin until molten (approximate viscosity on the order of 100,000cp), and then applying the hot molten polymer to a highly polished metalband or drum with an extrusion die, cooling the film, and finallypeeling the film from the metal support. For many reasons, however,films prepared by melt extrusion are generally not suitable for opticalapplications. Principal among these is the fact that melt extruded filmsexhibit a high degree of optical birefringence. In the case ofpolycarbonate polymer, there is the additional problem of melting thepolymer. Polycarbonate films have exceptionally high meltingtemperatures of approximately 230° C. and may require very highprocessing temperature in excess of 300° C. At these high temperatures,polycarbonates are vulnerable to hydrolysis and discoloration. For thesereasons, melt extrusion methods are generally not suitable forfabricating many resin films, including polycarbonate films intended foroptical applications. Rather, casting methods are generally used toproduce these films.

[0006] Resin films for optical applications are manufactured almostexclusively by casting methods. Casting methods involve first dissolvingthe polymer in an appropriate solvent to form a dope having a highviscosity on the order of 50,000 cp, and then applying the viscous dopeto a continuous, highly polished metal band or drum through an extrusiondie, partially drying the wet film, peeling the partially dried filmfrom the metal support, and conveying the partially dried film throughan oven to more completely remove solvent from the film. Cast filmstypically have a final dry thickness in the range of 40-200 μm. Ingeneral, thin films of less than 40 μm are very difficult to produce bycasting methods due to the fragility of wet film during the peeling anddrying processes. Films having a thickness of greater than 200 μm arealso problematic to manufacture due to difficulties associated with theremoval of solvent in the final drying step. Although the dissolutionand drying steps of the casting method add complexity and expense, castfilms generally have better optical properties when compared to filmsprepared by melt extrusion methods and problems associated with hightemperature processing are avoided.

[0007] Examples of optical films prepared by casting methods include:1.) Polyvinyl alcohol sheets used to prepare light polarizers asdisclosed in U.S. Pat. No. 4,895,769 to Land and U.S. Pat. No. 5,925,289to Cael as well as more recent disclosures in U.S. patent applicationSer. No. 2001/0039319 A1 to Harita and U.S. patent application Ser. No.2002/001700 A1 to Sanefuji, 2.) Cellulose triacetate sheets used forprotective covers for light polarizers as disclosed in U.S. Pat. No.5,695,694 to Iwata, 3.) Polycarbonate sheets used for protective coversfor light polarizers or for retardation plates as disclosed in U.S. Pat.No. 5,818,559 to Yoshida and U.S. Pat. Nos. 5,478,518 and 5,561,180,both to Taketani, and 4.) Polysulfone sheets used for protective coversfor light polarizers or for retardation plates as disclosed in U.S. Pat.Nos. 5,611,985 to Kobayashi and U.S. Pat. Nos. 5,759,449 and 5,958,305both to Shiro.

[0008] The manufacture of polycarbonate films by the casting method isconfounded by abrasion, scratch and wrinkle artifacts that may becreated during conveyance of the film as described in U.S. Pat. No.6,222,003 to Hosoi. These artifacts are created while the film passesover numerous conveyance rollers in the final drying and windingoperations of the casting method. To overcome these problems, cast filmsmay contain additives that act as lubricants, may be laminated with aprotective sheet, or may have the edges knurled to minimize damage tothe polycarbonate film. Alternatively, U.S. Pat. No. 6,222,003B1 toHosoi discloses a method of creating small irregularities on the surfaceof the cast polycarbonate film to minimize contact with the conveyancerollers and hence minimize scratching and wrinkling. These smallirregularities are said to be formed by the use of non-solvents in thecasting dope along with special drying conditions. However, lubricantsare known to compromise film clarity. Moreover, lamination and edgeknurling devices are expensive and add complexity to the castingprocess. Finally, the deliberate formation of surface irregularities ona film to be used for optical applications is complicated andundesirable. In general, optical films are preferred to be very smoothwith low haze.

[0009] Another disadvantage to the casting method is that cast filmshave significant optical birefringence. Although films prepared bycasting methods have lower birefringence when compared to films preparedby melt extrusion methods, birefringence remains objectionably high. Forexample, cellulose triacetate films prepared by casting methods exhibitin-plane retardation of 7 nanometers (nm) for light in the visiblespectrum as disclosed in U.S. Pat. No. 5,695,694 to Iwata. Apolycarbonate film prepared by the casting method is disclosed as havingan in-plane retardation of 17 nm in U.S. Pat. Nos. 5,478,518 and5,561,180 both to Taketani. U.S. patent application Ser. No.2001/0039319 A1 to Harita claims that color irregularities in stretchedpolyvinyl alcohol sheets are reduced when the difference in retardationbetween widthwise positions within the film is less than 5 nm in theoriginal unstretched film. For many applications of optical films, lowin-plane retardation values are desirable. In particular, values ofin-plane retardation of less than 10 nm are preferred.

[0010] Birefringence in cast films arises from orientation of polymersduring the manufacturing operations. This molecular orientation causesindices of refraction within the plane of the film to be measurablydifferent. In-plane birefringence is the difference between theseindices of refraction in perpendicular directions within the plane ofthe film. The absolute value of birefringence multiplied by the filmthickness is defined as in-plane retardation. Therefore, in-planeretardation is a measure of molecular anisotropy within the plane of thefilm.

[0011] During the casting process, molecular orientation may arise froma number of sources including shear of the dope in the die, shear of thedope by the metal support during application, shear of the partiallydried film during the peeling step, and shear of the free-standing filmduring conveyance through the final drying step. These shear forcesorient the polymer molecules and ultimately give rise to undesirablyhigh birefringence or retardation values. To minimize shear and obtainthe lowest birefringence films, casting processes are typically operatedat very low line speeds of 1-15 m/min as disclosed in U.S. Pat. No.5,695,694 to Iwata. Slower line speeds generally produce the highestquality films.

[0012] Low birefringence polycarbonate films are exceptionally difficultto manufacture. This is due to the fact that polycarbonates are rigidpolymers and readily align or orient when exposed to shear forces in thecasting process. While polycarbonate films have been prepared with lowin-plane retardation using a batch casting method, continuously castpolycarbonate films have objectionably high retardation. For example,although batch-cast polycarbonate films have been described within-plane retardation values of 4-8 nm, continuous-cast films areconsiderably higher at 17 nm as disclosed in U.S. Pat. Nos. 5,478,518and 5,561,180 both to Taketani. Batch casting is primarily a laboratorymethod for preparing short experimental samples for physical analysisand is not suitable for large-scale manufacture of polycarbonate films.

[0013] Another drawback to the casting method is the inability toaccurately apply multiple layers. As noted in U.S. Pat. No. 5,256,357 toHayward, conventional multi-slot casting dies create unacceptablynon-uniform films. In particular, line and streak non-uniformity isgreater than 5% with prior art devices. Acceptable two layer films maybe prepared by employing special die lip designs as taught in U.S. Pat.No. 5,256,357 to Hayward, but the die designs are complex and may beimpractical for applying more than two layers simultaneously.

[0014] Another drawback to the casting method is the restrictions on theviscosity of the dope. In casting practice, the viscosity of dope is onthe order of 50,000 cp. For example, U.S. Pat. No. 5,256,357 to Haywarddescribes practical casting examples using dopes with a viscosity of100,000 cp. In general, cast films prepared with lower viscosity dopesare known to produce non-uniform films as noted for example in U.S. Pat.No. 5,695,694 to Iwata. In U.S. Pat. No. 5,695,694 to Iwata, the lowestviscosity dopes used to prepare casting samples are approximately 10,000cp. At these high viscosity values, however, casting dopes are difficultto filter and degas. While fibers and larger debris may be removed,softer materials such as polymer slugs are more difficult to filter atthe high pressures found in dope delivery systems. Particulate andbubble artifacts create conspicuous inclusion defects as well as streaksand may create substantial waste.

[0015] In addition, the casting method can be relatively inflexible withrespect to product changes. Because casting requires high viscositydopes, changing product formulations requires extensive down time forcleaning delivery systems to eliminate the possibility of contamination.Particularly problematic are formulation changes involving incompatiblepolymers and solvents. In fact, formulation changes are so timeconsuming and expensive with the casting method that most productionmachines are dedicated exclusively to producing only one film type.

[0016] Finally, cast films may exhibit undesirable cockle or wrinkles.Thinner films are especially vulnerable to dimensional artifacts eitherduring the peeling and drying steps of the casting process or duringsubsequent handling of the film. In particular, the preparation ofcomposite optical plates from resin films requires a lamination processinvolving application of adhesives, pressure, and high temperatures.Very thin films are difficult to handle during this lamination processwithout wrinkling. In addition, many cast films may naturally becomedistorted over time due to the effects of moisture. For optical films,good dimensional stability is necessary during storage as well as duringsubsequent fabrication of composite optical plates.

SUMMARY OF THE INVENTION

[0017] It is therefore an object of the present invention to overcomethe limitations of prior art casting methods and provide a new coatingmethod for preparing amorphous polycarbonate films having very lowin-plane birefringence.

[0018] It is a further object of the present invention to provide a newmethod of producing highly uniform polycarbonate films over a broadrange of dry thicknesses.

[0019] Yet another object of the present invention is to provide amethod of preparing polycarbonate films by simultaneously applyingmultiple layers to a moving substrate.

[0020] Still another object of the present invention is to provide a newmethod of preparing polycarbonate films with improved dimensionalstability and handling ability by temporarily adhering the polycarbonatefilm to a supporting carrier substrate at least until it issubstantially dry and then subsequently separating the carrier substratefrom the polycarbonate film.

[0021] A further object of the present invention is to overcome thelimitations of the prior art casting method and define a new coatingmethod for preparing resin films without the need for co-solvents,lubricants, or protective laminates as converting aids to minimizescratch and abrasion artifacts.

[0022] Briefly stated, the foregoing and numerous other features,objects and advantages of the present invention will become readilyapparent upon review of the detailed description, claims and drawingsset forth herein. These features, objects and advantages areaccomplished by applying a low viscosity fluid containing polycarbonateresin onto a moving carrier substrate by a coating method. Thepolycarbonate film is not separated from the carrier substrate until thecoated film is substantially dry (≦10% residual solvent by weight). Infact, the composite structure of polycarbonate film and carriersubstrate may be wound into rolls and stored until needed. Thus, thecarrier substrate cradles the polycarbonate film and protects againstshearing forces during conveyance through the drying process. Moreover,because the polycarbonate film is dry and solid when it is finallypeeled from the carrier substrate, there is no shear or orientation ofpolymer within the film due to the peeling process. As a result,polycarbonate resin films prepared by the current invention areremarkably amorphous and exhibit very low in-plane birefringence.

[0023] Polycarbonate films can be made with the method of the presentinvention having a thickness of about 1 to 500 μm. Very thinpolycarbonate films of less than 40 microns can be easily manufacturedat line speeds not possible with prior art methods. The fabrication ofvery thin films is facilitated by a carrier substrate that supports thewet film through the drying process and eliminates the need to peel thefilm from a metal band or drum prior to a final drying step as requiredin the casting methods described in prior art. Rather, the polycarbonatefilm is substantially, if not completely, dried before separation fromthe carrier substrate. In all cases, dried polycarbonate films have aresidual solvent content of less than 10% by weight. In a preferredembodiment of the present invention, the residual solvent content isless than 5%, and most preferably less than 1%. Thus, the presentinvention readily allows for preparation of very delicate thin films notpossible with the prior art casting method. In addition, thick films ofgreater than 40 μm may also be prepared by the method of the presentinvention. To fabricate thicker films, additional coatings may beapplied over a film-substrate composite either in a tandem operation orin an offline process without comprising optical quality. In this way,the method of the present invention overcomes the limitation of solventremoval during the preparation of thicker films since the first appliedfilm is dry before application of a subsequent wet film. Thus, thepresent invention allows for a broader range of final film thicknessthan is possible with casting methods.

[0024] In the method of the present invention, polycarbonate films arecreated by forming a single or, preferably, a multi-layer composite on aslide surface of a coating hopper, the multi-layer composite including abottom layer of low viscosity, one or more intermediate layers, and anoptional top layer containing a surfactant, flowing the multi-layercomposite down the slide surface and over a coating lip of the coatinghopper, and applying the multi-layer composite to a moving substrate. Inparticular, the use of the method of the present invention is shown toallow for application of several liquid layers having uniquecomposition. Coating aids and additives may be placed in specific layersto improve film performance or improve manufacturing robustness. Forexample, multi-layer application allows a surfactant to be placed in thetop spreading layer where needed rather than through out the entire wetfilm. In another example, the concentration of polycarbonate in thelowermost layer may be adjusted to achieve low viscosity and facilitatehigh-speed application of the multi-layer composite onto the carriersubstrate. Therefore, the present invention provides an advantageousmethod for the fabrication of multiple layer composite films such asrequired for certain optical elements or other similar elements.

[0025] Wrinkling and cockle artifacts are minimized with the method ofthe present invention through the use of the carrier substrate. Byproviding a stiff backing for the polycarbonate film, the carriersubstrate minimizes dimensional distortion of the polycarbonate resinfilm. This is particularly advantageous for handling and processing verythin films of less than about 40 microns. Moreover, scratches andabrasion artifacts that are known to be created by the casting methodare avoided with the method of the present invention since the carriersubstrate lies between the polycarbonate film and potentially abrasiveconveyance rollers during all drying operations. Thus, the method of thepresent invention does not require the use of co-solvents, lubricants orprotective laminates as converting aids as are needed in castingoperations to minimize abrasion artifacts. In addition, the restrainingnature of the carrier substrate also eliminates the tendency ofpolycarbonate films to distort or cockle over time as a result ofchanges in moisture levels. Thus, the method of the current inventioninsures that polycarbonate films are dimensionally stable duringpreparation and storage as well as during final handling steps necessaryfor fabrication of optical elements.

[0026] In the practice of the method of the present invention it ispreferred that the substrate be a discontinuous sheet such aspolyethylene terephthalate (PET). The PET carrier substrate may bepretreated with a subbing layer or an electrical discharge device tomodify adhesion between the polycarbonate film and the PET substrate. Inparticular, a subbing layer or electrical discharge treatment mayenhance the adhesion between the film and the substrate, but still allowthe film to be subsequently peeled away from the substrate.

[0027] Although the present invention is discussed herein withparticular reference to a slide bead coating operation, those skilled inthe art will understand that the present invention can be advantageouslypracticed with other coating operations. For example, freestanding filmshaving low in-plane retardation should be achievable with single ormultiple layer slot die coating operations and single or multiple layercurtain coating operations. Moreover, those skilled in the art willrecognize that the present invention can be advantageously practicedwith alternative carrier substrates. For example, peeling films havinglow in-plane birefringence should be achievable with other resinsupports [e.g. polyethylene naphthalate (PEN), cellulose acetate, PET],paper supports, resin laminated paper supports, and metal supports (e.g.aluminum).

[0028] Practical applications of the present invention include thepreparation of polycarbonate sheets used for optical films, laminatefilms, release films, photographic films, and packaging films amongothers. In particular, polycarbonate sheets prepared by the method ofthe present invention may be utilized as optical films in themanufacture of electronic displays such as liquid crystal displays. Forexample, liquid crystal displays are comprised of a number of filmelements including polarizer plates, compensation plates and electrodesubstrates. Polarizer plates are typically a multi-layer compositestructure having dichroic film (normally stretched polyvinyl alcoholtreated with iodine) with each surface adhered to a protective cover.The polycarbonate films prepared by the method of the present inventionare suitable as protective covers for polarizer plates. Thepolycarbonate films prepared by the method of the present invention arealso suitable for the manufacture of compensation plates and electrodesubstrates.

[0029] The polycarbonate film produced with the method of the presentinvention is an optical film. As produced, the polycarbonate films madewith the method of the present invention will have a light transmittanceof at least about 85 percent, preferably at least about 90 percent, andmost preferably, at least about 95 percent. Further, as produced, thepolycarbonate film will have a haze value of less than 1.0 percent. Inaddition, the polycarbonate films are smooth with a surface roughnessaverage of less than 100 nm and most preferably with a surface roughnessof less than 50 nm

BRIEF DESCRIPTION OF THE DRAWINGS

[0030]FIG. 1 is a schematic of an exemplary coating and drying apparatusthat can be used in the practice of the method of the present invention.

[0031]FIG. 2 is a schematic of an exemplary coating and drying apparatusof FIG. 1 including a station where the polycarbonate web separated fromthe substrate is separately wound.

[0032]FIG. 3 is a schematic of an exemplary multi-slot coating apparatusthat can be used in the practice of the method of the present invention.

[0033]FIG. 4 shows a cross-sectional representation of a single-layerpolycarbonate film partially peeled from a carrier substrate and formedby the method of the present invention.

[0034]FIG. 5 shows a cross-sectional representation of a single-layerpolycarbonate film partially peeled from a carrier substrate and formedby the method of the present invention wherein the carrier substrate hasa subbing layer formed thereon.

[0035]FIG. 6 shows a cross-sectional representation of a multi-layerpolycarbonate film partially peeled from a carrier substrate and formedby the method of the present invention.

[0036]FIG. 7 shows a cross-sectional representation of a multi-layerpolycarbonate film partially peeled from a carrier substrate and formedby the method of the present invention wherein the carrier substrate hasa subbing layer formed thereon.

[0037]FIG. 8 is a schematic of a casting apparatus as used in prior artto cast polycarbonate films.

DETAILED DESCRIPTION OF THE INVENTION

[0038] Turning first to FIG. 1 there is shown a schematic of anexemplary and well known coating and drying system 10 suitable forpracticing the method of the present invention. The coating and dryingsystem 10 is typically used to apply very thin films to a movingsubstrate 12 and to subsequently remove solvent in a dryer 14. A singlecoating apparatus 16 is shown such that system 10 has only one coatingapplication point and only one dryer 14, but two or three (even as manyas six) additional coating application points with corresponding dryingsections are known in the fabrication of composite thin films. Theprocess of sequential application and drying is known in the art as atandem coating operation.

[0039] Coating and drying apparatus 10 includes an unwinding station 18to feed the moving substrate 12 around a back-up roller 20 where thecoating is applied by coating apparatus 16. The coated web 22 thenproceeds through the dryer 14. In the practice of the method of thepresent invention the final dry film 24 comprising a polycarbonate resinfilm on substrate 12 is wound into rolls at a wind-up station 26.

[0040] As depicted, an exemplary four-layer coating is applied to movingweb 12. Coating liquid for each layer is held in respective coatingsupply vessel 28, 30, 32, 34. The coating liquid is delivered by pumps36, 38, 40, 42 from the coating supply vessels to the coating apparatus16 conduits 44, 46, 48, 50, respectively. In addition, coating anddrying system 10 may also include electrical discharge devices, such ascorona or glow discharge device 52, or polar charge assist device 54, tomodify the substrate 12 prior to application of the coating.

[0041] Turning next to FIG. 2 there is shown a schematic of the sameexemplary coating and drying system 10 depicted in FIG. 1 with analternative winding operation. Accordingly, the drawings are numberedidentically up to the winding operation. In the practice of the methodof the present invention, the dry film 24 comprising a substrate (whichmay be a resin film, paper, resin coated paper or metal) with apolycarbonate coating applied thereto is taken between opposing rollers56, 58. The polycarbonate film 60 is peeled from substrate 12 with thepolycarbonate film going to winding station 62 and the substrate 12going to winding station 64. In a preferred embodiment of the presentinvention, polyethylene terephthalate (PET) is used as the substrate 12.The substrate 12 may be pretreated with a subbing layer to enhanceadhesion of the coated film 60 to the substrate 12.

[0042] The coating apparatus 16 used to deliver coating fluids to themoving substrate 12 may be a multi-layer applicator such as a slide beadhopper, as taught for example in U.S. Pat. No. 2,761,791 to Russell, ora slide curtain hopper, as taught by U.S. Pat. No. 3,508,947 to Hughes.Alternatively, the coating apparatus 16 may be a single layerapplicator, such as a slot die hopper or a jet hopper. In a preferredembodiment of the present invention, the application device 16 is amulti-layer slide bead hopper.

[0043] As mentioned above, coating and drying system 10 includes a dryer14 that will typically be a drying oven to remove solvent from thecoated film. An exemplary dryer 14 used in the practice of the method ofthe present invention includes a first drying section 66 followed byeight additional drying sections 68-82 capable of independent control oftemperature and air flow. Although dryer 14 is shown as having nineindependent drying sections, drying ovens with fewer compartments arewell known and may be used to practice the method of the presentinvention. In a preferred embodiment of the present invention the dryer14 has at least two independent drying zones or sections.

[0044] Preferably, each of drying sections 68-82 have independenttemperature and airflow controls. In each section, temperature may beadjusted between 5° C. and 150° C. To minimize drying defects from casehardening or skinning-over of the wet polycarbonate film, optimum dryingrates are needed in the early sections of dryer 14. There are a numberof artifacts created when temperatures in the early drying zones areinappropriate. For example, fogging or blush of polycarbonate films isobserved when the temperature in zones 66, 68 and 70 are set at 25° C.This blush defect is particularly problematic when high vapor pressuresolvents (methylene chloride and acetone) are used in the coatingfluids. Aggressively high temperatures are also associated with otherartifacts such as case hardening, reticulation patterns and microvoidsin the polycarbonate film. In a preferred embodiment of the presentinvention, the first drying section 66 is operated at a temperature ofat least about 25° C. but less than 95° C. with no direct airimpingement on the wet coating of the coated web 22. In anotherpreferred embodiment of the method of the present invention, dryingsections 68 and 70 are also operated at a temperature of at least about25° C. but less than 95 ° C. It is preferred that initial dryingsections 66, 68 be operated at temperatures between about 30° C. andabout 60° C. It is most preferred that initial drying sections 66, 68 beoperated at temperatures between about 30° C. and about 50° C. Theactual drying temperature in drying sections 66, 68 may be optimizedempirically within these ranges by those skilled in the art.

[0045] Referring now to FIG. 3, a schematic of an exemplary coatingapparatus 16 is shown in detail. Coating apparatus 16, schematicallyshown in side elevational cross-section, includes a front section 92, asecond section 94, a third section 96, a fourth section 98, and a backplate 100. There is an inlet 102 into second section 94 for supplyingcoating liquid to first metering slot 104 via pump 106 to thereby form alowermost layer 108. There is an inlet 110 into third section 96 forsupplying coating liquid to second metering slot 112 via pump 114 toform layer 116. There is an inlet 118 into fourth section 98 forsupplying coating liquid to metering slot 120 via pump 122 to form layer124. There is an inlet 126 into back plate 100 for supplying coatingliquid to metering slot 128 via pump 130 to form layer 132. Each slot104, 112, 120, 128 includes a transverse distribution cavity. Frontsection 92 includes an inclined slide surface 134, and a coating lip136. There is a second inclined slide surface 138 at the top of secondsection 94. There is a third inclined slide surface 140 at the top ofthird section 96. There is a fourth inclined slide surface 142 at thetop of fourth section 98. Back plate 100 extends above inclined slidesurface 142 to form a back land surface 144. Residing adjacent thecoating apparatus or hopper 16 is a coating back up roller 20 aboutwhich a web 12 is conveyed. Coating layers 108, 116, 124, 132 form amulti-layer composite which forms a coating bead 146 between lip 136 andsubstrate 12. Typically, the coating hopper 16 is movable from anon-coating position toward the coating backing roller 20 and into acoating position. Although coating apparatus 16 is shown as having fourmetering slots, coating dies having a larger number of metering slots(as many as nine or more) are well known and may be used to practice themethod of the present invention.

[0046] In the method of the present invention, the coating fluids arecomprised principally of a polycarbonate resin dissolved in an organicsolvent. Polymers of the polycarbonate type are available in a varietyof molecular weights as well as in numerous permutations around thebasic molecular structure. Common to all polycarbonates are thecarbonate linkages and usually the presence of stabilizing phenyl groups(Ph) in the polymer backbone. In terms of commercially significantpolycarbonates, the condensation product of the dihydridic phenol,2,2-bis-(4-hydroxyphenyl)-propane (Bisphenol-A), with a carbonateprecursor, such as phosogene or diphenyl carbonate, forms a polymerhaving recurring units of -O-Ph-C(CH₃)₂-Ph-O-CO-. Polycarbonates of theBisphenol-A type are both readily available and relatively inexpensive.Less readily available and more expensive are the numerous polycarbonatecopolymers that may be formed by the addition of various dihydric phenolderivatives during polymer synthesis. Examples of such derivatives are1,1-bis-(4-hydroxyphenyl)cyclohexane (Bisphenol Z),1,1-bis-(4-hydroxyphenyl)-3,3,5-trimethyl cyclohexane,2,2-bis-(3-methyl-4-hydroxyphenyl)propane (Bisphenol C),1,1-bis-(4-hydroxyphenyl)-1-phenyl ethane (Bisphenol P),bis-(4-hydroxyphenyl)-diphenyl methane, among others. These co-polymericpolycarbonates may be formulated to alter material properties such asthermal stability, impact resistance and the like, while maintaininggood optical properties. In the method of the present invention, thereare no particular restrictions as to the type of polycarbonate or blendof polycarbonate co-polymers used to form a film. Polycarbonate resinsare commercially available from General Electric and Bayer.

[0047] In terms of organic solvents for polycarbonates, suitablesovlents include, for example, chlorinated solvents (methylene chlorideand 1,2 dichloroethane), alcohols (methanol, ethanol, n-propanol,isopropanol, n-butanol, isobutanol, diacetone alcohol, phenol, andcyclohexanol), ketones (acetone, methylethyl ketone, methylisobutylketone, and cyclohexanone), esters (methyl acetate, ethyl acetate,n-propyl acetate, isopropyl acetate, isobutyl acetate, and n-butylacetate), aromatics (toluene and xylenes) and ethers (tetrahydrofuran,1,3-dioxolane, 1,2-dioxolane, 1,3-dioxane, 1,4-dioxane, and1,5-dioxane). Polycarbonate solutions may be prepared with a blend ofthe aforementioned solvents. Preferred primary solvents includemethylene chloride and 1,3-dioxolane. Preferred co-solvents includetoluene, tetrahydrofuran, cyclohexanone, methanol, ethanol, andisopropanol.

[0048] Coating fluids may also contain small amounts of plasticizers.Appropriate plasticizers for polycarbonate films include phthalateesters (diethylphthalate, dibutylphthalate, dicyclohexylphthalate,dioctylphthalate, didecylphthalate and butyl octylphthalate), adipateesters (dioctyl adipate), carbonates (dicetyl carbonate and distearylcarbonate) and phosphate esters (tricresyl phosphate and triphenylphosphate). Plasticizers are normally used to improve the flowcharacteristics of polycarbonates processed by the melt extrusionmethod. However, plasticizers may be used here as coating aids in theconverting operation to minimize premature film solidification at thecoating hopper and to improve drying characteristics of the wet film. Inthe method of the present invention, plasticizers may be used tominimize blistering, curl and delamination of polycarbonate films duringthe drying operation. In a preferred embodiment of the presentinvention, plasticizers may be added to the coating fluid at a totalconcentration of up to 5% by weight relative to the concentration ofpolymer in order to mitigate defects in the final polycarbonate film.

[0049] Coating fluids may also contain surfactants as coating aids tocontrol artifacts related to flow after coating. Artifacts created byflow after coating phenomena include mottle, repellencies, orange-peel(Bernard cells), and edge-withdraw. Surfactants used control flow aftercoating artifacts include siloxane and fluorochemical compounds.Examples of commercially available surfactants of the siloxane typeinclude: 1.) Polydimethylsiloxanes such as DC200 Fluid from Dow Corning,2.) Poly(dimethyl, methylphenyl)siloxanes such as DC510 Fluid from DowCorning, and 3.) Polyalkyl substituted polydimethysiloxanes such asDC190 and DC 1248 from Dow Corning as well as the L7000 Silwet series(L7000, L7001, L7004 and L7230) from Union Carbide, and 4.) Polyalkylsubstituted poly(dimethyl, methylphenyl)siloxanes such as SF1023 fromGeneral Electric. Examples of commercially available fluorochemicalsurfactants include: 1.) Fluorinated alkyl esters such as the Fluoradseries (FC430 and FC431) from the 3M Corporation, 2.) Fluorinatedpolyoxyethylene ethers such as the Zonyl series (FSN, FSN100, FSO,FSO100) from Du Pont, 3.) Acrylate:polyperfluoroalkyl ethylacrylatessuch as the F series (F270 and F600) from NOF Corporation, and 4.)Perfluoroalkyl derivatives such as the Surflon series (S383, S393, andS8405) from the Asahi Glass Company. In the method of the presentinvention, surfactants are generally of the non-ionic type. In apreferred embodiment of the present invention, non-ionic compounds ofeither the siloxane or fluorinated type are added to the uppermostlayers.

[0050] In terms of surfactant distribution, surfactants are mosteffective when present in the uppermost layers of the multi-layercoating. In the uppermost layer, the concentration of surfactant ispreferably 0.001-1.000% by weight and most preferably 0.010-0.500%. Inaddition, lesser amounts of surfactant may be used in the seconduppermost layer to minimize diffusion of surfactant away from theuppermost layer. The concentration of surfactant in the second uppermostlayer is preferably 0.000-0.200% by weight and most preferably between0.000 -0.100% by weight. Because surfactants are only necessary in theuppermost layers, the overall amount of surfactant remaining in thefinal dried film is small.

[0051] Although surfactants are not required to practice the method ofthe current invention, surfactants do improve the uniformity of thecoated film. In particular, mottle nonuniformities are reduced by theuse of surfactants. In transparent polycarbonate films, mottlenonuniformities are not readily visualized during casual inspection. Tovisualize mottle artifacts, organic dyes may be added to the uppermostlayer to add color to the coated film. For these dyed films,nonuniformities are easy to see and quantify. In this way, effectivesurfactant types and levels may be selected for optimum film uniformity.

[0052] Turning next to FIGS. 4 through 7, there are presentedcross-sectional illustrations showing various film configurationsprepared by the method of the present invention. In FIG. 4, asingle-layer polycarbonate film 150 is shown partially peeled from acarrier substrate 152. Polycarbonate film 150 may be formed either byapplying a single liquid layer to the carrier substrate 152 or byapplying a multiple layer composite having a composition that issubstantially the same among the layers. Alternatively in FIG. 5, thecarrier substrate 154 may have been pretreated with a subbing layer 156that modifies the adhesive force between the single layer polycarbonatefilm 158 and the substrate 154. FIG. 6 illustrates a multiple layer film160 that is comprised of four compositionally discrete layers includinga lowermost layer 162 nearest to the carrier support 170, twointermediate layers 164, 166, and an uppermost layer 168. FIG. 6 alsoshows that the entire multiple layer composite 160 may be peeled fromthe carrier substrate 170. FIG. 7 shows a multiple layer composite film172 comprising a lowermost layer 174 nearest to the carrier substrate182, two intermediate layers 176, 178, and an uppermost layer 180 beingpeeled from the carrier substrate 182. The carrier substrate 182 hasbeen treated with a subbing layer 184 to modify the adhesion between thecomposite film 172 and substrate 182. Subbing layers 156 and 184 may becomprised of a number of polymeric materials such as polyvinylbutyrals,cellulosics, acrylics, gelatin and poly(acrylonitrile-co-vinylidenechloride-co-acrylic acid). The choice of materials used in the subbinglayer may be optimized empirically by those skilled in the art.

[0053] The method of the present invention may also include the step ofcoating over a previously prepared composite of polycarbonate film andcarrier substrate. For example, the coating and drying system 10 shownin FIGS. 1 and 2 may be used to apply a second multi-layer film to anexisting polycarbonate film/substrate composite. If the film/substratecomposite is wound into rolls before applying the subsequent coating,the process is called a multi-pass coating operation. If coating anddrying operations are carried out sequentially on a machine withmultiple coating stations and drying ovens, then the process is called atandem coating operation. In this way, thick films may be prepared athigh line speeds without the problems associated with the removal oflarge amounts of solvent from a very thick wet film. Moreover, thepractice of multi-pass or tandem coating also has the advantage ofminimizing other artifacts such as streak severity, mottle severity, andoverall film nonuniformity.

[0054] The practice of tandem coating or multi-pass coating requiressome minimal level of adhesion between the first-pass film and thecarrier substrate. In some cases, film/substrate composites having pooradhesion are observed to blister after application of a second or thirdwet coating in a multi-pass operation. To avoid blister defects,adhesion must be greater than 0.3 N/m between the first-passpolycarbonate film and the carrier substrate. This level of adhesion maybe attained by a variety of web treatments including various subbinglayers and various electronic discharge treatments. However, excessiveadhesion between the applied film and substrate is undesirable since thefilm may be damaged during subsequent peeling operations. In particular,film/substrate composites having an adhesive force of greater than 250N/m have been found to peel poorly. Films peeled from such excessively,well-adhered composites exhibit defects due to tearing of the filmand/or due to cohesive failure within the film. In a preferredembodiment of the present invention, the adhesion between thepolycarbonate film and the carrier substrate is less than 250 N/m. Mostpreferably, the adhesion between polycarbonate film and the carriersubstrate is between 0.5 and 25 N/m.

[0055] The method of the present invention is suitable for applicationof polycarbonate resin coatings to a variety of substrates such aspolyethylene terephthalate (PET), polyethylene naphthalate (PEN),polystyrene, and other polymeric films. Polymeric substrates may beunstretched, uniaxially stretched or biaxially stretched prior toapplication of the polycarbonate coatings. Additional substrates mayinclude paper, laminates of paper and polymeric films, glass, cloth,aluminum and other metal supports. In some cases, substrates may bepretreated with subbing layers or electrical discharge devices.Substrates may also be pretreated with functional layers containingvarious binders and addenda.

[0056] The prior art method of casting resin films is illustrated inFIG. 8. As shown in FIG. 8, a viscous polymeric dope is deliveredthrough a feed line 200 to an extrusion hopper 202 from a pressurizedtank 204 by a pump 206. The dope is cast onto a highly polished metaldrum 208 located within a first drying section 210 of the drying oven212. The cast film 214 is allowed to partially dry on the moving drum208 and is then peeled from the drum 208. The cast film 214 is thenconveyed to a final drying section 216 to remove the remaining solvent.The final dried film 218 is then wound into rolls at a wind-up station220. The prior art cast film typically has a thickness in the range offrom 40 to 200 μm.

[0057] Coating methods are distinguished from casting methods by theprocess steps necessary for each technology. These process steps in turnaffect a number of tangibles such as fluid viscosity, converting aids,substrates, and hardware that are unique to each method. In general,coating methods involve application of dilute low viscosity liquids tothin flexible substrates, evaporating the solvent in a drying oven, andwinding the dried film/substrate composite into rolls. In contrast,casting methods involve applying a concentrated viscous dope to a highlypolished metal drum or band, partially drying the wet film on the metalsubstrate, stripping the partially dried film from the substrate,removing additional solvent from the partially dried film in a dryingoven, and winding the dried film into rolls. In terms of viscosity,coating methods require very low viscosity liquids of less than 5,000cp. In the practice of the method of the present invention the viscosityof the coated liquids will generally be less than 2000 cp and most oftenless than 1500 cp. Moreover, in the method of the present invention theviscosity of the lowermost layer is preferred to be less than 200 cp.and most preferably less than 100 cp. for high speed coatingapplication. In contrast, casting methods require highly concentrateddopes with viscosity on the order of 10,000-100,000 cp for practicaloperating speeds. In terms of converting aids, coating methods generallyinvolve the use of surfactants as converting aids to control flow aftercoating artifacts such as mottle, repellencies, orange peel, and edgewithdraw. In contrast, casting methods do not require surfactants.Instead, converting aids are only used to assist in the stripping andconveyance operations in casting methods. For example, lower alcoholsare sometimes used as converting aids in cast polycarbonate films tominimize abrasion artifacts during conveyance through drying ovens. Interms of substrates, coating methods generally utilize thin (10-250micron) flexible supports. In contrast, casting methods employ thick(1-100 mm), continuous, highly polished metal drums or rigid bands. As aresult of these differences in process steps, the hardware used incoating is conspicuously different from those used in casting as can beseen by a comparison of the schematics shown in FIGS. 1 and 8,respectively.

[0058] The advantages of the present invention are demonstrated by thefollowing practical examples given below. In these examples, thepolycarbonate (PC) was the Bisphenol-A homopolymer with a weight averagemolecular weight of 54,000 daltons as determined with polystyreneequivalent weight distributions using size exclusion chromatography.

EXAMPLE 1

[0059] This example describes the single pass formation of a very thinpolycarbonate film. The coating apparatus 16 illustrated in FIG. 1 wasused to apply four liquid layers to a moving substrate 12, 170 ofuntreated polyethylene terephthalate (PET) to form a single layer filmas illustrated earlier in FIG. 6. The substrate speed was 25 cm/s. Allcoating fluids were comprised of PC dissolved in methylene chloride. Thelowermost layer 162 had a viscosity of 17 cp. and a wet thickness of 14μm on the moving substrate 170. The second 164 and third 166 layers eachhad a viscosity of 660 cp. and had a combined final wet thickness of 27μm on the moving substrate 170. In addition, the third layer 166 alsocontained a fluorinated surfactant (Surflon S8405) at concentration of0.02 %. The uppermost layer 168 had a viscosity of 107 cp. and a wetthickness of 22 μm on the moving substrate 170. The uppermost layer 168also contained a fluorinated surfactant (Surflon S8405) at a weightpercent of 0.10%. Coatings were applied at a temperature of 16° C. Thegap between the coating lip 136 and the moving substrate 12 (see FIG. 3)was 200 μm. The pressure differential across the coating bead 146 wasadjusted between 0-10 cm of water to establish a uniform coating. Thetemperature in the drying sections 66 and 68 was 40° C. The temperaturein the drying section 70 was 50° C. The temperature in the dryingsections 72, 74, 76, 78, 80 was 120° C. The temperature in the dryingsection 82 was 25° C. The composite of PC film and PET substrate waswound into rolls. When peeled from the untreated PET substrate, thefinal dry film had a thickness of 10 μm. The peeled PC film was freefrom scratch and wrinkle artifacts and had an in-plane retardation ofless than 5.0 nm. Properties of this polycarbonate film are summarizedin Table I.

EXAMPLE 2

[0060] This example describes the single pass formation of a thin PCfilm. The conditions were identical to those described in Example 1except that the combined wet thickness of the second and third layers164 and 166 was increased to 73 μm. The composite of PC film and PETsubstrate was wound into rolls. When peeled from the subbed PETsubstrate, the final dry film had a thickness of 20 μm. The peeled PCfilm had a good appearance, was smooth, was free from scratch andwrinkles artifacts, and had an in-plane retardation of less than 5.0 nm.Properties of this PC film are summarized in Table I.

EXAMPLE 3

[0061] This example describes the single pass formation of a thin PCfilm. The conditions were identical to those described in Example 1except that the combined wet thickness of the second and third layers164 and 166 was increased to 120 μm. The composite of PC film and PETsubstrate was wound into rolls. When peeled from the subbed PETsubstrate, the final dry film had a thickness of 30 μm. The PC film hada good appearance, was smooth, was free from scratch and wrinkleartifacts, and had an in-plane retardation of less than 5.0 nm.Properties of this PC film are summarized in Table I.

EXAMPLE 4

[0062] This example describes the single pass formation a PC film. Theconditions were identical to those described in Example 1 except thatthe combined wet thickness of the second and third layers 164 and 166was increased to 166 μm. The composite of PC film and PET substrate waswound into rolls. When peeled from the subbed PET substrate, the finaldry film had a thickness of 40 μm. The peeled PC film had a goodappearance, was smooth, was free from scratch and wrinkle artifacts, andhad an in-plane retardation of less than 5.0 nm. Properties of this PCfilm are summarized in Table I.

EXAMPLE 5

[0063] This example describes the formation of a thin PC film using twopass coating operation. The conditions were identical to those describedin Example 1 except that the wound composite of PC film and PETsubstrate of Example 1 was subsequently over-coated with an additionalpass. The second pass was conducted with the combined wet thickness ofthe second and third layers at 27 μm as described in Example 1. Thecomposite of PC film and PET substrate was wound into rolls. When peeledfrom the untreated PET substrate, the final dry film had a thickness of20 μm. The peeled PC film had a good appearance, was smooth, was freefrom scratch and wrinkle artifacts, and had an in-plane retardation ofless than 5.0 nm. Properties of this polycarbonate film are summarizedin Table I.

EXAMPLE 6

[0064] This example describes the formation of a PC film using athree-pass coating operation. The conditions were identical to thosedescribed in Example 2 except that the wound composite of PC film andPET substrate of Example 2 was subsequently over-coated with twoadditional passes. Each additional pass was conducted with the combinedwet thickness of the second and third layers at 73 μm as described inExample 2. The final composite of PC film and PET substrate was woundinto rolls. The final dry film had a thickness of 60 μm. The peeled PCfilm was smooth, was free from scratch and wrinkle artifacts, and had anin-plane retardation of less than 5.0 nm. Properties of this PC film aresummarized in Table I.

EXAMPLE 7

[0065] This example describes the formation of a PET/PC composite havingoptimal peeling properties. In this example, the PET support has asubbing layer applied to the coated side. The subbing layer ispolyvinylbutyral (˜12% vinyl alcohol content) having a dry thickness of10 μm and a surfactant content of 500 mg/sq-m of Surflon S-8405. Thispolyvinylbutyral layer is adhered to the subbed PET substrate.Otherwise, the conditions were identical to those described in Example2. The final composite of PC film and subbed PET substrate was woundinto rolls. The final dry film had a thickness of 20 μm. When peeledfrom the subbed PET substrate, the PC film was found to separate verysmoothly from the carrier support. The average adhesive strength of thePC film to the subbed PET substrate was found to be 1.8 N/m with astandard deviation of 0.4 N/m. This smooth peeling process contrastednoticeably with the more hesitant peeling properties of the untreatedPET substrate described earlier in Example 2. For untreated PETsubstrate of Example 2, the average adhesive strength of the PC film tothe subbed PET substrate was found to be 3.0 N/m with a higher standarddeviation of 1.4 N/m. This feature of smooth peeling is reflected in thesmaller standard deviation values found with the adhesion measurements.Similar results where observed with thicker PC films of 40 microns. For40 micron PC films prepared under the conditions of Example 4, samplesprepared using untreated PET and the polyvinylbutyral subbed PETdescribed here in Example 7 had standard deviations of adhesive strengthof 2.5 and 0.1 N/m, respectively. The PET substrate treated with thepolyvinylbutyral subbing layer displayed very smooth peelcharacteristics having very low standard deviation values of adhesivestrength.

COMPARATIVE EXAMPLE 1

[0066] This example describes the formation of a PET/PC composite havingpoor peeling properties. In this example, the PET support has a subbinglayer of poly(acrylonitrile-co-vinylidene chloride-co-acrylic acid) witha dry coverage of 100 mg/sq-m. Otherwise, the conditions for ComparativeExample 1 were identical to those described in Example 1. The final dryfilm had a thickness of 20 μm. When dried, the PC film could not bepeeled from the subbed PET substrate. For this composite film, theadhesive strength of the PC film to the subbed PET substrate was greaterthan 250 N/m.

COMPARATIVE EXAMPLE 2

[0067] This example describes defects formed as a result of poor dryingconditions during a single pass operation. The conditions forComparative Example 2 were identical to those described in Example 2except that the drying conditions were adjusted such that thetemperature in the first three drying zones 66, 68, 70 was decreased to25° C. When peeled from the subbed PET substrate, the final dry film hada thickness of 20 μm. The peeled PC film was of unacceptable quality dueto fogging of the film.

COMPARATIVE EXAMPLE 3

[0068] This example describes defects formed as a result of poor dryingconditions during a single pass operation. The conditions forComparative Example 3 were identical to those described in Example 2except that the drying conditions were adjusted such that thetemperature in the first three drying zones 66, 68, 70 was increased to95° C. When peeled from the subbed PET substrate, the final dry film hada thickness of 20 μm. The peeled PC film was of unacceptable quality dueto a reticulation pattern in the film as well as to blister artifacts.TABLE I Example Thickness Retardation Transmittance Haze Roughness 1 10μm 2.0 nm 92.1% 1.0% 1.3 nm 2 20 3.8 92.0 1.0 1.0 3 30 2.5 92.3 0.8 0.94 40 2.8 92.3 0.7 0.7 5 20 3.8 92.0 0.6 1.1 6 60 4.5 92.1 0.8 0.7

[0069] The following tests were used to determine the film propertiesgiven in Table I.

[0070] Thickness. Thickness of the final peeled film was measured inmicrons using a Model EG-225 gauge from the Ono Sokki Company.

[0071] Retardation. In-plane retardation (R_(e)) of peeled films weredetermined in nanometers (nm) using a Woollam M-2000V SpectroscopicEllipsometer at wavelengths from 370 to 1000 nm. In-plane retardationvalues in Table I are computed for measurements taken at 590 nm.In-plane retardation is defined by the formula:

R _(e) =|n _(x) −n _(y) |×d

[0072] where R_(e) is the in-plane retardation at 590 nm, n_(x) is theindex of refraction of the peeled film in the slow axis direction, n_(y)is the index of refraction of the peeled film in the fast axisdirection, and d is the thickness of the peeled film in nanometers (nm).Thus, R_(e) is the absolute value of the difference in birefringencebetween the slow axis direction and the fast axis direction in the planeof the peeled film multiplied by the thickness of the film.

[0073] Transmittance and Haze. Total transmittance and haze are measuredusing the Haze-Gard Plus (Model HB-4725) from BYK-Gardner. Totaltransmittance is all the light energy transmitted through the film asabsorbed on an integrating sphere. Transmitted haze is all light energyscattered beyond 2.5° as absorbed on an integrating sphere.

[0074] Surface Roughness. Surface roughness was determined in nanometers(nm) by scanning probe microscopy using TappingMode™ Atomic ForceMicroscopy (Model D300 from Digital Instruments).

[0075] Adhesion. The adhesion strength of the coated samples wasmeasured in Newtons per meter (N/m) using a modified 180° peel test withan Instron 1122 Tensile Tester with a 500 gram load cell. First, 0.0254m (one inch) wide strips of the coated sample were prepared.Delamination of the coating at one end was initiated using a piece of 3MMagic Tape. An additional piece of tape was then attached to thedelaminated part of the coating and served as the gripping point fortesting. The extending tape was long enough to extend beyond the supportsuch that the Instron grips did not interfere with the testing. Thesample was then mounted into the Instron 1122 Tensile Tester with thesubstrate clamped in the upper grip and the coating/tape assemblyclamped in the bottom grip. The average force (in units of Newtons)required to peel the coating off the substrate at a 180° angle at speedof 2 inches/min (50.8 mm/min) was recorded. Using this force value theadhesive strength in units of N/m was calculated using the equation:

S _(A) =F _(p)(1−cos θ)/w

[0076] wherein S_(A) is the adhesive strength, F_(p) is the peel force,θ is the angle of peel (180°), and w is the width of the sample (0.0254m).

[0077] Residual Solvent. A qualitative assessment of residual solventsremaining in a dried film is done by first peeling the film from thecarrier substrate, weighing the peeled film, incubating the film in anoven at 150° C. for 16 hours, and finally weighing the incubated film.Residual solvent is expressed as percentage of the weight differencedivided by the post-incubation weight.

[0078] From the foregoing, it will be seen that this invention is onewell adapted to obtain all of the ends and objects hereinabove set forthtogether with other advantages which are apparent and which are inherentto the apparatus.

[0079] It will be understood that certain features and subcombinationsare of utility and may be employed with reference to other features andsubcombinations. This is contemplated by and is within the scope of theclaims.

[0080] As many possible embodiments may be made of the invention withoutdeparting from the scope thereof, it is to be understood that all matterherein set forth and shown in the accompanying drawings is to beinterpreted as illustrative and not in a limiting sense.

PARTS LIST:

[0081] 10 coating and drying system 12 moving substrate/web 14 dryer 16coating apparatus 18 unwinding station 20 back-up roller 22 coated web24 dry film 26 wind up station 28 coating supply vessel 30 coatingsupply vessel 32 coating supply vessel 34 coating supply vessel 36 pumps38 pumps 40 pumps 42 pumps 44 conduits 46 conduits 48 conduits 50conduits 52 discharge device 54 polar charge assist device 56 opposingrollers 58 opposing rollers 60 polycarbonate film 62 winding station 64winding station 66 diying section 68 drying section 70 drying section 72diying section 74 drying section 76 drying section 78 dtying section 80drying section 82 drying section 92 front section 94 second section 96third section 98 fourth section 100 back plate 102 inlet 104 meteringslot 106 pump 108 lower most layer 110 inlet 112 2nd metering slot 114pump 116 layer 118 inlet 120 metering slot 122 pump 124 form layer 126inlet 128 metering slot 130 pump 132 layer 134 incline slide surface 136coating lip 138 2nd incline slide surface 140 3rd incline slide surface142 4th incline slide surface 144 back land surface 146 coating bead 150polycarbonate film 152 carrier substrate 154 carrier substrate 156subbing layer 158 polycarbonate film 160 multiple layer film 162 lowermost layer 164 intermediate layers 166 intermediate layers 168 uppermost layer 170 carrier support 172 composite film 174 lower most layer176 intermediate layers 178 intermediate layers 180 upper most layers182 carrier substrate 184 subbing layer 200 feed line 202 extrusionhopper 204 pressurized tank 206 pump 208 metal drum 210 drying section212 drying oven 214 cast film 216 final drying section 218 final driedfilm 220 wind up station

What is claimed is:
 1. A coating method for forming a polycarbonate filmcomprising the steps of: (a) applying a liquid polycarbonate/solventmixture onto a moving, discontinuous carrier substrate; and (b) dryingthe liquid polycarbonate/solvent mixture to substantially remove thesolvent yielding a composite of a polycarbonate film adhered to thediscontinuous carrier substrate, the polycarbonate film being releasablyadhered to the discontinuous carrier substrate thereby allowing thepolycarbonate film to be peeled from the discontinuous carriersubstrate.
 2. A coating method as recited in claim 1 wherein: the liquidpolycarbonate/solvent mixture is applied using slide bead coating diewith a multi-layer composite being formed on a slide surface thereof. 3.A coating method as recited in claim 2 wherein: the viscosity of eachliquid layer of the multi-layer composite is less than 5000 cp.
 4. Acoating method as recited in claim 1 wherein: the carrier substrate ispolyethylene terephthalate.
 5. A coating method as recited in claim 1wherein: the carrier substrate has a subbing layer applied to the coatedsurface.
 6. A coating method as recited in claim 2 wherein: an uppermostlayer of the multi-layer composite contains a surfactant.
 7. A coatingmethod as recited in claim 1 wherein: the first drying section isoperated at a temperature between 25 and 95° C.
 8. A coating method asrecited in claim 1 further comprising the step of: winding the compositeinto at least one roll before the polycarbonate sheet is peeled from thediscontinuous carrier substrate.
 9. A coating method as recited in claim1 further comprising the steps of: (a) separating the polycarbonate filmfrom the carrier substrate immediately after the drying step; and (b)winding the polycarbonate film into at least one roll.
 10. A coatingmethod as recited in claim 8 further comprising the step of: (a)unwinding at least a portion of at least one roll of the composite; and(b) separating the polycarbonate film from the carrier substrate.
 11. Acoating method as recited in claim 1 wherein: the polycarbonate film isadhered to the carrier substrate with an adhesive strength of less thanabout 250 N/m.
 12. A coating method as recited in claim 9 furthercomprising the step of: reducing residual solvent in the polycarbonatefilm to less than 10% by weight prior to the separating step.
 13. Acoating method as recited in claim 10 further comprising the step of:reducing residual solvent in the polycarbonate film to less than 10% byweight prior to the separating step.
 14. A coating method as recited inclaim 8 further comprising the step of: delivering the composite to auser of the polycarbonate film, the carrier substrate acting as aprotective support for the polycarbonate film prior to the polycarbonatefilm being separated from the substrate carrier.
 15. A coating method asrecited in claim 1 further comprising the step of: including aplasticizer in the liquid polycarbonate/solvent mixture.
 16. A coatingmethod as recited in claim 1 wherein: the polycarbonate film has anin-plane retardation of less than 20 nm.
 17. A coating method as recitedin claim 1 wherein: the polycarbonate film has an in-plane retardationof less than 10 nm.
 18. A coating method as recited in claim 1 wherein:the polycarbonate film has an in-plane retardation of less than 5.0 nm.19. A coating method as recited in claim 1 further comprising the stepof: applying at least one additional polycarbonate layer to thecomposite after the drying step.
 20. A coating method as recited inclaim 1 wherein: the polycarbonate film has a thickness in the range of1 to 500 μm.
 21. A composite film comprising: a polycarbonate filmcoated on a discontinuous carrier substrate, the polycarbonate filmhaving a thickness in the range of from about 1 to about 500 μm, thepolycarbonate film having an in-plane retardation that is less than 20nm, the polycarbonate film being adhered to the carrier substrate withan adhesive strength of less than about 250 N/m.
 22. A composite film asrecited in claim 21 wherein: the polycarbonate film has an in-planeretardation that is less than 10 nm.
 23. A composite film as recited inclaim 21 wherein: the polycarbonate film has an in-plane retardationthat is less than 5.0 nm.
 24. A composite film as recited in claim 21wherein: the polycarbonate film is adhered to the carrier substrate withan adhesive strength of at least about 0.3 N/m.
 25. A composite film asrecited in claim 21 wherein: the polycarbonate film is peelable from thecarrier substrate.
 26. A composite film as recited in claim 21 wherein:the polycarbonate film is a multi-layer composite.
 27. A composite filmas recited in claim 26 wherein: at least a top layer of the multi-layercomposite includes a surfactant therein.
 28. A composite film as recitedin claim 21 wherein: a plasticizer is incorporated in the polycarbonatefilm.
 29. A polycarbonate film made by the method of claim 1 wherein:the in-plane retardation is less than 20 nm.
 30. A polycarbonate filmcomprising: a layer of polycarbonate formed by a coating operation, thepolycarbonate film having a thickness in the range of from about 1 toabout 500 μm, the polycarbonate film having an in-plane retardation thatis less than 20 nm.
 31. A polycarbonate film as recited in claim 30further comprising: a plasticizer incorporated in the polycarbonatefilm.
 32. A polycarbonate film as recited in claim 30 wherein: thepolycarbonate film having an in-plane retardation that is less than 10nm.
 33. A polycarbonate film as recited in claim 30 wherein: thepolycarbonate film having an in-plane retardation that is less than 5.0nm.
 34. A coating method as recited in claim 1 further comprising thestep of: using the polycarbonate film to form a light polarizer.
 35. Adisplay device including a polycarbonate film therein made by the methodof claim
 1. 36. A polycarbonate film comprising: a layer ofpolycarbonate formed by a coating operation, the polycarbonate filmhaving an in-plane retardation that is less than about 20 nm.
 37. Apolycarbonate film as recited in claim 36 wherein: the polycarbonatefilm having an in-plane retardation that is less than 10 nm.
 38. Apolycarbonate film as recited in claim 36 wherein: the polycarbonatefilm having an in-plane retardation that is less than 5.0 nm.
 39. Acomposite film as recited in claim 26 wherein: only a top layer of themulti-layer composite includes a surfactant therein.
 40. A compositefilm as recited in claim 26 wherein: at least a top layer of themulti-layer composite includes a fluorinated surfactant therein.
 41. Acoating method as recited in claim 2 wherein: an uppermost layer of themulti-layer composite contains a fluorinated surfactant.
 42. A coatingmethod as recited in claim 2 wherein: an uppermost layer of themulti-layer composite contains a polysiloxane surfactant.
 43. Anelectronic display device having view screen comprising: a polycarbonatefilm having an in-plane retardation that is less than about 10 nm. 44.An electronic display device as recited in claim 43 wherein: thepolycarbonate film having an in-plane retardation that is less thanabout 5 nm.
 45. An polycarbonate film comprising: a coated layer ofpolycarbonate having an in-plane retardation that is less than about 20nm.
 46. A coating method as recited in claim 7 wherein: the drying stepis initially performed at a temperature in the range of from about 25°C. to less than 95° C.
 47. A coating method as recited in claim 7wherein: the drying step is initially performed at a temperature in therange of from about 30° C. to about 60° C.
 48. A coating method asrecited in claim 7 wherein: the drying step is initially performed at atemperature in the range of from about 30° C. to about 50° C.
 49. Anelectronic display device as recited in claim 43 wherein: thepolycarbonate film has a light transmittance of at least about 85percent and a haze value of less than about 1.0 percent.
 50. Apolycarbonate film as recited in claim 45 wherein: the coated layer ofpolycarbonate has a light transmittance of at least about 85 percent anda haze value of less than about 1.0 percent.
 51. A coating method asrecited in claim 1 wherein: the optical resin film has a lighttransmittance of at least about 85 percent and a haze value of less thanabout 1.0 percent.
 52. A coating method as recited in claim 1 wherein:the optical resin film has an average surface roughness of less thanabout 100 nm.
 53. A coating method as recited in claim 1 wherein: theoptical resin film has an average surface roughness of less than about50 nm.
 54. A coating method as recited in claim 1 wherein: the opticalresin film has an average surface roughness of not more than about 1 nm.